text: /*=========================================================================
  
    Program:   Visualization Toolkit
    Module:    vtkSobelGradientMagnitudePass.h
  
    Copyright (c) Ken Martin, Will Schroeder, Bill Lorensen
    All rights reserved.
    See Copyright.txt or http://www.kitware.com/Copyright.htm for details.
  
       This software is distributed WITHOUT ANY WARRANTY; without even
       the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
       PURPOSE.  See the above copyright notice for more information.
  
  =========================================================================*/
  /**
   * @class   vtkSobelGradientMagnitudePass
   * @brief   Implement a post-processing edge
   * detection with a Sobel gradient magnitude render pass.
   *
   * Detect the edges of the image renderered by its delegate. Edge-detection
   * uses a Sobel high-pass filter (3x3 kernel).
   *
   * This pass expects an initialized depth buffer and color buffer.
   * Initialized buffers means they have been cleared with farest z-value and
   * background color/gradient/transparent color.
   * An opaque pass may have been performed right after the initialization.
   *
   * The delegate is used once.
   *
   * Its delegate is usually set to a vtkCameraPass or to a post-processing pass.
   *
   * This pass requires a OpenGL context that supports texture objects (TO),
   * framebuffer objects (FBO) and GLSL. If not, it will emit an error message
   * and will render its delegate and return.
   *
   * @par Implementation:
   * To compute the gradient magnitude, the x and y components of the gradient
   * (Gx and Gy) have to be computed first. Each computation of Gx and Gy uses
   * a separable filter.
   * The first pass takes the image from the delegate as the single input
   * texture.
   * The first pass has two outputs, one for the first part of Gx, Gx1, result of
   * a convolution with (-1 0 1), one for the first part of Gy, Gy1, result of a
   * convolution with (1 2 1).
   * The second pass has two inputs, Gx1 and Gy1. Kernel (1 2 1)^T is applied
   * to Gx1 and kernel (-1 0 1)^T is applied to Gx2. It gives the values for
   * Gx and Gy. Those values are then used to compute the magnitude of the
   * gradient which is stored in the render target.
   * The gradient computation happens per component (R,G,B). A is arbitrarily set
   * to 1 (full opacity).
   *
   * @par Implementation:
   * \image html vtkSobelGradientMagnitudePassFigure.png
   * \image latex vtkSobelGradientMagnitudePassFigure.eps
   *
   * @sa
   * vtkRenderPass
  */
  
  #ifndef vtkSobelGradientMagnitudePass_h
  #define vtkSobelGradientMagnitudePass_h
  
  #include "vtkRenderingOpenGL2Module.h" // For export macro
  #include "vtkImageProcessingPass.h"
  
  class vtkDepthPeelingPassLayerList; // Pimpl
  class vtkOpenGLFramebufferObject;
  class vtkOpenGLHelper;
  class vtkOpenGLRenderWindow;
  class vtkTextureObject;
  
  
  class VTKRENDERINGOPENGL2_EXPORT vtkSobelGradientMagnitudePass : public vtkImageProcessingPass
  {
  public:
    static vtkSobelGradientMagnitudePass *New();
    vtkTypeMacro(vtkSobelGradientMagnitudePass,vtkImageProcessingPass);
    void PrintSelf(ostream& os, vtkIndent indent) override;
  
    /**
     * Perform rendering according to a render state \p s.
     * \pre s_exists: s!=0
     */
    void Render(const vtkRenderState *s) override;
  
    /**
     * Release graphics resources and ask components to release their own
     * resources.
     * \pre w_exists: w!=0
     */
    void ReleaseGraphicsResources(vtkWindow *w) override;
  
   protected:
    /**
     * Default constructor. DelegatePass is set to NULL.
     */
    vtkSobelGradientMagnitudePass();
  
    /**
     * Destructor.
     */
    ~vtkSobelGradientMagnitudePass() override;
  
    /**
     * Graphics resources.
     */
    vtkOpenGLFramebufferObject *FrameBufferObject;
    vtkTextureObject *Pass1; // render target for the scene
    vtkTextureObject *Gx1; // render target 0 for the first shader
    vtkTextureObject *Gy1; // render target 1 for the first shader
  
    // Structures for the various cell types we render.
    vtkOpenGLHelper *Program1; // shader to compute Gx1 and Gy1
    vtkOpenGLHelper *Program2; // shader to compute |G| from Gx1 and Gy1
  
   private:
    vtkSobelGradientMagnitudePass(const vtkSobelGradientMagnitudePass&) = delete;
    void operator=(const vtkSobelGradientMagnitudePass&) = delete;
  };
  
  #endif
  